Device for the stereophonic reproduction of signals



A ril 18, 1967 P. w. ZWOLLO DEVICE FOR THE STEREOPHONIC REPRODUCTION OF SIGNALS Filed April 16, 1965 J 5 a 3 T 23 T 45 5 23 J an 15000123000 39000 53000 H INVENTOR. PE TRUS W.ZWOLLO w E. AGEN United States Patent 3,315,038 DEVICE FOR THE STEREOPHONIC REPRODUCTION OF SIGNALS Petrus Willem Zwollo, Emmasingel, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Apr. 16, 1965, Ser. No. 448,603 Claims priority, application Netherlands, Apr. 25, 1964, 6,404,602 4 Claims. (Cl. 179-15) The invention relates to a device for the stereophonic reproduction of signals, supplied in the form of the sum signal A+B of the coherent stereophonic signals A and B and of the difference signal A--B of said coherent stereophonic signals, transmitted by amplitude-modulation of a subcarrier frequency with suppression of the carrier, in common with a pilot signal of half the carrier frequency, which device comprises at the input end, the parallel combination of an information channel passing the stereophonic signals and a pilot channel passing solely the pilot signal and comprising a frequency doubler, for the local production of the subcarrier frequency, whilst the information channel and the pilot signal channel are connected, for restoring the coherent stereophonic signals A and B, the diagonal points of a ring which comprises, in each branch, the series combination of a resistor and a rectifier, the rectifiers having, around the ring, the same pass directions, whereas the coherent signals A and B are derived from output resistors connected to the further diagonal points of the ring.

In the copending United States patent applications Ser. No. 271,292, filed Apr. 8, 196-3 and Ser. No. 401,377, filed Oct. 5, 1964 such stereophonic reproducing devices are described, in which in accordance with the fact whether a stereophonic signal or a monaural signal is received, automatic changeover to stereophonic or monaural reproduction respectively is carried out. From elaborate tests the applicant has now found that the monaural reproduction quality, particularly of monaural signals of low amplitudes, did not come up completely to expectations.

The invention has for its object to provide a device of the type described above, in which the monaural reproduction quality is considerably improved in a surprisingly simple manner.

The device according to the invention is characterised in that the junction of the series resistor and the rectifier in a ring branch and the corresponding junction in the opposite ring branch are connected each through a separate resistor to the terminal of a bias voltage source, the other terminal of which is connected through the two output resistors, as connections passing direct current, to the diagonal points of the ring connected to said resistors, said bias voltage source provides for all rectifiers of the ring a substantially identical bias adjustment in the for ward direction.

The invention and its advantage will now be described more fully with reference to the figures.

FIG. 1 shows a stereophonic receiver comprising a de vice according to the invention;

FIG. 2 shows a frequency diagram and FIG. 3 shows a detailed diagram for explaining the operation of the device according to the invention.

The stereophonic receiver shown in FIG. 1 is intended for the reception of signals transmitted by frequency modulation of the same carrier frequency, said signals being formed by the sum signal A+B of the coherent stereophonic signals A and B, for example in the band from 30 to 15,000 c./s. and the difference signal A-B transmitted with carrier suppression on a subcarrier frequency of 38 kc./s. by amplitude-modulation and by a pilot signal of 19 kc./s., the modulation signal thus formed in the band from 30 to 53,000 c./s. modulating in frequency the carrier frequency with a sweep of kc./s. at a maxlmum.

The stereophony receiver comprises an aerial 1, an intermediate-frequency stage 2 with a mixing stage and an oscillator 3, connected thereto, the intermediate frequency oscillations obtained by mixing in the band from 10,700 kc./ s. being supplied, subsequent to amplification and inhibition, if necessary, in the intermediate frequency stage 2, to an output filter which is formed by two coupled circuits 4, 5.

The bandpass filter 4, 5 forms part of a type of frequency detector known for the detection of normal frequency modulation transmissions and comprises two rectifiers 6, 7, which are connected to the ends of the circuit 5 and which pass current in opposite directions, and which are connected to an output impedance 8, whilst a centre tap of the circuit 5 is connected to the end of a coupling coil 9, connected to the circuit coil 4. The output impedance 8 of the frequency detector is formed by a resistor 10, which is shunted by the series combination of two capacitors 11, 12 with an earth connected junction and a smoothing capacitor 13, whilst the output voltage of the frequency detector is derived from the centre tap of the resistor 10.

At the output impedance of the frequency detector 6, 7 .there appears an output voltage of the waveform shown in FIG. 2 in the frequency diagram. It will be seen from the figure that it is composed of the sum signal A+B in the band from 30 to 15,000 c./s., the difference signal A-B in the band from 23,000 to 53,000 c,/s. modulating the subcarrier of 38 kc./s. with carrier suppression and the pilot signal of 19,000 c./s.

In order .to restore the coherent stereophonic signals A and B from the output voltage of the frequency detector 6, 7 of FIG. 2 these signals are supplied to a device comprising, at the input end, the parallel combination of an information channel 14 passing the output voltage of the frequency detector and a pilot channel -15, passing solely the pilot signal and comprising a frequency doubler 20 for producing the subcarrier frequency of 38 kc./ s. In the embodiment shown the pilot channel comprises an input pilot filter '16 and a transistor 17, connected as a pilot signal amplifier, the output circuit including a pilot signal filter formed by two coupled circuits 18, 19, the output voltage of which is fed to the frequency doubler 20, which is formed by a push-pull rectifier comprising rectifiers 21, .22 and an output resistor 23.

To the output circuit of the frequency doubler 20 there is connected a transistor 24, connected as an amplifier, the collector circuit of which includes a circuit 25, tuned to the subcarrier frequency of 38 kc./s., and an indicator 45, formed by a direct cur-rent meter. The: local carrier oscillation obtained from the frequency doubler 20 is fed, subsequent to amplification in the transistor 24, through the circuit 25 for further processing to a coupling coil 26, whilst the indicator 45 deflects to indicate the presence of a local carrier oscillation.

The information channel 14 is connected to the centre tap of the coupling coil 26 through a low bandpass filter 27, allowing only the information signals to pass and in order to restore the coherent stereophonic signals A and .B the ends of the coupling coil 26 are connected to the diagonal points of a ring 46, each of the ring branches including the series combination of a series resistor 32, 3-3, 3 4, 35 and a rectifier 28, 29, 30, 31, the latter having the same pass directions in the course of the circuit 46. The two rectifiers 28, 29 and the two rectifiers 30, 31 con stitute each an electronic switch, the output Voltage of the two electronic switches thus formed 23, '29 and 30, 31 being derived from output resistors"36, 36', located at diagonal points of the ring 46.

The coherent stereophonic signals A and B are derived from the output resistors 36, 36 of the two electronic switches 28, 29 and 30, 31 and are fed to reproducing devices 39, 40 through low frequency amplifiers located in separate reproducing channels. The low frequency amplifiers are constructed in an identical manner and are provided with transistors 41, 42 the emitter circuits including non-shunted resistors 43, 44, whilst the amplified coherent signals A and B are derived from output impedances 50, 51, included in the collector circuits of the transistors 41, 42, said irnpedances being connected, as is shown diagrammatically in the figure, through separating capacitors 47, 48 to the reproducing devices 39, 40. Cross talk between the reproducing channels 37, 38 is compensated for in the device shown in a simple manner by means of a transverse resistor 49, connected between the emitter circuits 43, 44 of the transistors 41, 42, said resistor being adjustable for the cross talk compensation. A cross talk of better than 40 db was realized in this case, which may be considered to be excellent for stereophonic reproduction.

The operation of the device described above will now be explained more :fully.

When the stereophonic signal illustrated in FIG. 2 appears at the output circuit of the frequency detector 6, 7 it is fed through the information channel 14 to the centre tap of the coupling coil 26, whilst in the pilot channel 15 the pilot signal selected in the circuit 16 and amplified in the pilot amplifier 17 is fed, subsequent to frequency doubling in the frequency doubler 2,0, as a local subcarrier frequency to the electronic switches 28, 29 and 30, 31, said frequency serving as a switching signal for the electronic switches 28, 29, 30 and 31.

' If the phase of the local subcarrier frequency at the coupling coil 26 corresponds accurately with the phase of the carrier frequency associated with the incoming signal, which phase adjustment may be carried out in a simple manner by detuning one of the circuits 18, 19 of the pilot filter in the output circuit of the pilot amplifier 17, the coherent stereophonic signals A and B appear across the output resistors of the electronic switches 28, 29, 30, 31, which are rendered conducting in successive half periods by the local subcarrier frequency operating as a switching signal, so that a sampling value of the incoming information signal is obtained, during the consecutive half periods, at the output resistors 36, 36' of the electronic switches 28, 29, 30, 31, which value is formed at one output resistor by the sum of the sampling value of the A-l-B signal and the value of the AB signal during the positive half period of the subcarrier frequency and at the outer resistor by the sum of the sampling value of the A+B signal and the value of the AB signal during the negative half period of the subcarrier frequency. Smoothing of these sampling values by means of output impedances 50, 51 in the output circuits of the low-frequency amplifiers 41, 42, forming smoothing filters, supplies the coherent stereophonic signals A and B and these smoothing filters 50, 51, which may, if desired, operate simultaneously as de-emphasis networks, suppress the pilot signal of 19 kc./s. coming in through the information channel 14, as well as further detection residues.

At the appearance of a stereophonic input signal of the waveform shown in FIG. 2 the device so far described supplies a stereophonic reproduction of excellent quality, and at a monaural input signal for example in the frequency band from 30 to 15,000 c./ s a monaural reproduction is obtained. In this case the pilot channel 15 does not provide a local carrier frequency and the monaural input signal is directly transmitted through the electronic switches 28, 29, 30 and 31 to the separate reproducing channels 37, 38. In accordance with the polarity of the input signal of each of the two electronic switches 28, 29, 30 and 31, one of the rectifiers is conducting. Wit-h an input signal of positive polarity, for example the rectifiers 29, 30 of the ring 46 are conducting, whereas the rectifiers 28, 31 are cut off and conversely with an input signal of negative polarity the rectifiers 29, 30 are cut off and the rectifiers 28, 31 are conducting. Through the alternately conducting rectifiers 28, 29, 30, 31 the monaural input signal is transmitted to the output impedances formed by the resistors 36, 36' of the electronic switches 28, 29, 30, 31.

Apart from the simplicity of construction of the device described so far offers the possibility of obtaining a stereophonic or monaural reproduction without mechanical change-over according as a sterophonic or a monaural input respectively is received, whilst the indicator 45 provides an indication whether a stereophonic or a monaural input signal is received. The applicant has found, however, that the reproduction quality of a monaural input signal, particularly in the case of low amplitude, does not come up to expectation, which was found to be due to the non-linear effects at the instants of change-over, i.e. the instants when the transmission of the monaural signal from one pair of rectifiers 29, 30 or 28, 31 is taken over by the other pair of rectifiers 28, 31 or 29, 30 respectively. The distortions of the monaural signal produced by said non-linear effects are for example of the order of a few percent.

Whilst maintaining the excellent reproduction quality of the stereophonic input signal, the reproduction quality of the monaural signal is improved, inaccordance with the invention, in a surprisingly simple manner to a considerable extent by connecting the junction 52 of the series resistor 34 and the rectifier 30 in one ring branch and the corresponding junction 53 in the opposite ring branch 29, 33 each through a separate resistor 54 and 55 respectively to the terminal 56 of the supply voltage source, the earthconnected other terminal 57 of which is connected through the two output resistors 36, 36', forming connections passing direct current, to the diagonal points of the ring 46,

connected to said output resistors 36, 36. The direct voltage source supplies for all rectifiers 28, 29, 30, 31 of the ring 46 a substantially identical bias adjustment in the pass direction.

For illustrating the variations of the direct currents FIG. 3 shows the ring 46 in the inverse position; identical elements are designated by the same reference numerals. The direct curent from the voltage source terminal 56 passing through the resistors 54, 55 will thus be distributed among the resistors 33 and 34 and the circuits including the rectifiers 29, 31 and 30, 28 respectively, it passing subsequently through the output resistors 36 and 36' to the terminal 57 of the voltage source, whilst the direct current in the two circuits including the rectifiers 29, 31 and 30, 28 provides a pre-adjustment of the rectifiers 29, 31 and 30, 28 in the pass direction. Owing to the symmetry a substantially identical preadjustment in the pass direction of all rectifiers 29, 31, 30, 28 is obtained, which can be set in a simple manner to the desired value by rendering the two resistors 54, 55 adjustable in common.

When a stereophonic signal is applied to the device according to the invention, the rectifiers 28, 29, 30 and 31 operating as electronic switches will be alternately by the local carrier fed to the coupling coil 26, during each half period and cut off in the manner described above, so that the coherent stereophonic signals A and B can be derived through the alternately released electronic switches 28, 29, 30, 31 from the output resistors 36 and 36'. A direct current flows not only through the resistors 33, 34 but also through the two rectifiers of the conducting electronic switches 28, 29, 30, 31 and through the coupling coil 26, said direct currents being identical for the two rectifiers of each of the electronic switches 28, 29, 30 and 31 owing to the symmetry of said switches. Owing to the identical pre-adjustment of the rectifiers 28,, 29, 30, 31 the stereophonic reproduction is not adversely affected in any respect and as stated above, this reproduction is of excellent quality.

When a monaural input signal is fed to the centre tap of the coupling coil 26, the rectifiers 28, 31 are conducting in the manner described above with positive polarity of the monaural input signal and the rectifiers 29, 30 are conducting with negative polarity. Owing to the pre-adjustment of the rectifiers in the pass direction the distortions produced by the non-linear effects at the change-over are considerably reduced. At the instants when one pair of rectifiers 28, 31 or 29, 30- respectively is cut off, the other pair of rectifiers 29, 30 or 28, 3 1 respectively is already in the conducting state by the pre-adjustment in the pass direction. The improvement obtained is considerable. When by the common adjustment of the resistors 54, 55 the value of the pre-adjustment of the rectifiers 28, 29, 30 is set, it is found that with a given value of the setting current a fairly sharp minimum appears in the distortion level of the monaural signal reproduction; this distortion level has dropped by a factor of about 10. With the rectifiers employed of the AA119 type the minimum distortion level was found to lie at a setting current Of 3/,ua.

Apart from the excellent stereophonic reproduction quality, the particular simple construction and the possibilities of adjustment, the device according to the invention is distinguished by a considerable improvement in the reproduction quality of monaural signals, whilst it has furthermore been found that owing to the preadjustment of the rectifiers 28, 29, 30, 31 a given amount of compensation of the stray of rectifier characteristics is obtained.

For a practically tested device of the type described above the following data are given;

Rectifiers 28, 29, 30, 31 4XAA119 Resistors 54, 55 mohms 1.5

Resistors 32, 33, 34, 35 kohms Output resistors 36, 36' kohms 47 Supply voltage v 15 What is claimed is:

1. A device for the stereophonic reproduction of signals supplied in the form of the sum signal A-l-B of coherent stereophonic signals A and B and of the difference signal A-B of said coherent stereophonic signals, amplitude modulating a subcarrier frequency with. carrier suppression in common with a pilot signal of half the carrier frequency to the device, which comprise at the input end the parallel combination of an information channel passing the incoming stereophonic signals and a pilot channel passing solely the pilot signal, comprising a frequency doubler for the local production of the carrier frequency, the information channel and the pilot channel being connected, in order to restore the stereophonic signals A and B, to diagonal points of a ring comprisin in each branch the series combination of a resistor and a rectifier, the rectifiers having the same pass direction in the course of the ring, whilst the coherent signals A and B are derived from output resistors connected to the further diagonal points of the ring, characterized in that the junction of the series resistor and the rectifier in one ring branch and the corresponding junction in the opposite ring branch are connected each through a separate resistor to the terminal of a bias voltage source and in that the other terminal of the bias voltage source is connected through the two output resistors forming connections passing direct currents to the diagonal points of the ring connected to said output resistors, said bias voltage source providing a substantially identical preadjustment of all rectifiers of the ring in the pass direction.

2. A device as claimed in claim 1, characterized in that the two resistors lying between the terminal of the bias voltage source and the junction of the rectifier and the series resistor in the opposite ring branches are adjustable in common.

3. A demodulator circuit for stereophonic signals of the type comprising a sum signal of first and second coherent stereophonic signals and a difference signal of said first and second signals amplitude modulated on a suppressed subcarrier wave, said demodulator circuit comprising a source of said stereophonic signals, a source of oscillations of the frequency of said subcarrier wave, a bridge circuit, each arm of said bridge circuit comprising a seriesconnected resistor and rectifier, said rectifiers being poled to pass current in the same direction around said bridge circuit, means applying said oscillations with opposite phase to a first pair of diagonal junctions of said bridge circuit, first output impedance means connected between one of the remaining junctions of said bridge circuit and a point of reference potential, second output impedance means connected between the other remaining junction of said bridge circuit and said point, means applying said stereophonic signals between said point and said first pair of junctions with the same phase, bias means having one terminal connected to said point, and separate resistor means connecting the other terminal of said bias means to separate junctions between resistors and rectifiers of a pair of opposite arms of said bridge circuit, said bias means providing substantially identical forward bias for all rectifiers of said bridge circuit.

4. A demodulator circuit for stereophonic signals of the type comprising a sum signal of first and second coherent stereophonic signals and a difference signal of said first and second signals amplitude modulated on a suppressed subcarrier wave, said demodulator circuit comprising a source of said stereophonic signals, a source of oscillations of the frequency of said subcarrier wave, a bridge circuit, each arm of said bridge circuit comp-rising a series-connected resistor and rectifier with an electrode of the rectifier of one arm being connected to a first junction of said bridge circuit and an electrode of the rectifier of the opposite arm being connected to a second diagonally opposite junction of said bridge circuit, said rectifiers being poled to pass current in the same direction around said bridge circuit, means applying said oscillations with opposite phase between a first pair of opposite junctions of said bridge circuit, means applying said stereophonic signals between a point of reference potential and said first pair of junctions with the same phase, first output resistor means connected between said point and a junction of the remaining pair of junctions of said bridge circuit, second output resistor means connected between said point and the other junction of the remaining pair of junctions of said bridge circuit, a source of bias voltage having one terminal connected to said point, third resistor means connected between the other terminal of said bias source and a junction of the resistor and rectifier in said one arm or said bridge circuit, and fourth resistor means connected between said other terminal and the junction of the resistor and rectifier in said opposite arm of said bridge circuit, whereby a substantially identical forward bias is applied to each rectifier of said bridge circuit.

References Cited by the Examiner UNITED STATES PATENTS 3,167,615 1/1965 Wilhelm et al 17915 DAVID G. REDI'NBAUGH, Primary Examiner. R. L. GRIFFIN, Assistant Examiner. 

3. A DEMODULATOR CIRCUIT FOR STEREOPHONIC SIGNALS OF THE TYPE COMPRISING A SUM SIGNAL OF FIRST AND SECOND COHERENT STEREOPHONIC SIGNALS AND A DIFFERENCE SIGNAL OF SAID FIRST AND SECOND SIGNAL AMPLITUDE MODULATED ON A SUPPRESSED SUBCARRIER WAVE, SAID DEMODULATOR CIRCUIT COMPRISING A SOURCE OF SAID STEREOPHONIC SIGNALS, A SOURCE OF OSCILLATIONS OF THE FREQUENCY OF SAID SUBCARRIER WAVE, A BRIDGE CIRCUIT, EACH ARM OF SAID BRIDGE CIRCUIT COMPRISING A SERIESCONNECTED RESISTOR AND RECTIFIER, SAID RECTIFIERS BEING POLED TO PASS CURRENT IN THE SAME DIRECTION AROUND SAID BRIDGE CIRCUIT, MEANS APPLYING SAID OSCILLATIONS WITH OPPOSITE PHASE TO A FIRST PAIR OF DIAGONAL JUNCTIONS OF SAID BRIDGE CIRCUIT, FIRST OUTPUT IMPEDANCE MEANS CONNECTED BETWEEN ONE OF THE REMAINING JUNCTIONS OF SAID BRIDGE CIRCUIT AND A POINT OF REFERENCE POTENTIAL, SECOND OUTPUT IMPEDANCE MEANS CONNECTED BETWEEN THE OTHER REMAINING JUNCTION OF SAID BRIDGE CIRCUIT AND SAID POINT, MEANS APPLYING SAID STEREOPHONIC SIGNALS BETWEEN SAID POINT AND SAID FIRST PAIR OF JUNCTIONS WITH THE SAME PHASE, BIAS MEANS HAVING ONE TERMINAL CONNECTED TO SIAD POINT, AND SEPARATE RESISTOR MEANS CONNECTING THE OTHER TERMINAL OF SAID BIASMEANS TO SEPARATE JUCTIONS BETWEEN RESISTORS AND RECTIFIERS OF A PAIR OF OPPOSITE ARMS OF SAID BRIDGE CIRCUIT, SAID BIAS MEANS PROVIDING SUBSTANTIALLY IDENTICAL FORWARD BIAS FOR ALL RECTIFIERS OF SAID BRIDGE CIRCUIT. 